1. Field of the Invention:
This invention generally relates to reducing the moisture content of low-quality solid carbonaceous fuels, such as low-rank coals and the like, and more particularly to an improved process for drying such materials so that they will not re-adsorb substantial moisture when stored or transported.
Low-rank coals, such as lignite and sub-bituminous coal, usually contain relatively large quantities of water (i.e., about 10 to 50% by weight). This factor makes the economics of transporting and combusting these coals considerably more expensive than for higher rank coals. Furthermore, this high moisture content makes low-rank coals dangerous owing to the possibility of spontaneous combustion during transportation or storage. Conventional drying processes prior to transportation or storage do not solve this problem because the coal will usually regain all of or most of the moisture from the atmosphere over a relatively short period of time (one to two weeks). In some cases the readsorption of moisture causes the coal to become even more pyrophoric than prior to drying. However, because of the generally low sulfur content of these low-rank coals, their continued and increasing use may be unavoidable, owing to the increasingly stringent regulations on sulfur emissions from coal combusting installations. Therefore, there is a need for an inexpensive method for beneficiation of low-rank coals to remove moisture and improve transportation and storage characteristics which obviate or substantially reduce the above problems.
2. Description of the Prior Art:
Various solutions have been proposed to address these problems outlined above. Many of these involve the use of very high temperature and pressures. The capital and energy costs associated with high pressure processes generally make them economically undesirable. For example, the Bureau of Mines process is performed at 1,500 psig, while the Koppelman processes described (for example in U.S. Pat. No. 4,052,168) require pressures of 1,000-3,000 psi with the higher pressures being preferable. These high pressure requirements also severely reduce the flexibility of these processes and increase the inherent risks and dangers associated therewith.
Also, the processes of the prior art require that the matter to be dried be subjected to the aforementioned high temperature and pressures for prolonged periods of time (referred to as residence times). For example, the Koppelman processes disclose usual residence times of from 15 minutes to one hour. These extended residence times not only increase the amount of energy input into the system, but also reduce the amount of product which can be processed over a given period of time, thereby further rendering those processes economically undesirable.
Further, such processes require specialized and expensive equipment, apparatuses, and facilities which increase capital investment and production costs, thereby further rendering those processes economically undesirable.
More recent approaches involve the use of lower temperatures and pressures, and various heat exchange gases to effect a heat transfer between the gas and the coal.
For example, U.S. Pat. No. 3,985,516, which issued 12 Oct. 1976 in the name of C. A. Johnson, teaches a process of drying low-rank coal using a warm inert (i.e. containing less than about 2% of oxygen) gas in a fluidized bed. Specific examples of the inert gas are nitrogen, carbon dioxide and flue gas. The drying gas temperatures disclosed are in the range of 250.degree. F. up to the volatilization temperature of the coal, and preferably in the range of 400.degree. to 500.degree. F., at atmospheric pressure. The dry coal is then passivated against re-absorption of moisture by coating the coal with a heavy liquid hydrocarbon material. Clearly, there is no teaching or suggestion that the drying conditions per se result in irreversibly dried coal.
Also, U.S. Pat. No. 4,810,258, which issued 7 Mar. 1989 in the name of M. M. Greene, teaches the use of a superheated gaseous drying medium to irreversibly dry low-rank coals. Although it is apparent that steam is the preferred gaseous medium, nitrogen is also specifically referenced. There is also provision for the re-cycling of combustion gases back to the drying area, so that a mixture of various drying gases is involved. The temperature and pressure of the drying medium is sufficient to heat the coal to temperatures in the range of 300.degree. to 450.degree. F. The preferred temperature and pressure of the gaseous medium is 850.degree. F., and 0.541 psi, respectively. Further, U.S. Pat. No. 5,035,721, which issued 30 Jul. 1991 to L. Atherton, is quite similar to ,258 in that is also uses a superheated inert (gas or steam) de-moisturizing medium. The only specific drying temperature disclosed in 850.degree. F., at a pressure of 0.541 psi.
Also, U.S. Pat. Nos. 4,705,533 and 4,800,015 in the name of J. J Simmons disclose a drying process for low-rank coals, in which the coal is immersed in oil before the heating step. During the heating process (300.degree.-450.degree. F.), the oil penetrates the coal particles, replacing the expended moisture, and preventing re-absorption of moisture. The oil coating also protects the resulting material from oxidation and spontaneous combustion. There is no teaching or suggestion of the use of a gaseous drying medium.
It will be appreciated that in the Prior Art processes where oil is not used in the drying process e.g. Greene and Atherton, the drying temperatures and the inherent energy requirements still need to be quite high in order to achieve substantially irreversible drying of the low-rank coal. This is evident in Johnson, who uses lower temperatures, but then has to coat the coal with oil to avoid re-absorption of moisture.
When oil is used in the drying process along with lower temperatures as in Simmons, there is a requirement to immerse the coal in oil. Subsequently, the excess oil employed has to be removed. In the current invention coal my be beneficiated prior to drying by using an oil agglomeration process. In this approach small amounts of oil (0.5 to 2 w/w %) are added to a vigorously mixed slurry of the coal particles. This method results in uniform distribution of controlled amounts of oil on the coal particle surfaces.